Which predictive algorithm can be used to predict a number given other numbers?
Impossible to solve until you define an error measurement ( by example $|R-R'|$ or $(R-R')^2$ ) and how this error changes when A, B and C change.
Extreme example: assume $R()$ is random (unrelated to A, B, C values) but static (always same $R(A,B,C)$ for same values of A,B,C). Given some values of A, B, C, you can only answer the value of $R(A,B,C)$ when A,B,C was in the training set. $R(A,B,C)$ is undefined and no predictable when A,B,C was not in the training set.
Moreover, improvements can be done if $R()$ has some properties, by example, if it is possible to state that $R(A,B,C)=R(B,A,C)$ or that $R(A1,B1,C1)=R(A2,B2,C2)$ if $A1+B1+C1=A2+B2+C2$.